1. Field of the Invention
The present invention relates to a method, apparatus and plant for desalinating saltwater. More particularly, the present invention relates to a method, apparatus and plant for desalinating saltwater by utilizing the energy difference that exists between two solutions of different solute concentrations that are separated by an ion exchange membrane.
2. Background of the Invention
Over one quarter of Earth's population does not have adequate access to freshwater. Inadequate access to freshwater is detrimental, as it can lead to disease and malnutrition, limit agricultural development, and inhibit economic growth.
In contrast to freshwater, however, saltwater is readily available. Saltwater in the form of seawater constitutes about 97% of the water on Earth. Unless seawater is sufficiently desalinated, though, it is not only undrinkable, but unsuitable for agriculture. “Desalination” refers to the process of removing anions and cations from saltwater. Seawater typically has a salt concentration of about 3.5% by mass; that is, about 35 grams of dissolved salt per liter of water. In contrast, drinkable water typically has a salt concentration of, at most, about 0.04%.
Several desalination methods are currently known in the art. One of the most popular methods at present is reverse osmosis (“RO”). RO involves mechanically forcing saltwater through spirally wound, semi-permeable membranes at high pressure. The membranes filter salt from the saltwater. Saltwater that is filtered using RO requires extensive pre-treatment, which increases RO's energy requirements. RO also suffers from performance issues when the temperature of the saltwater is over about 30° C., which can be the case when the saltwater source is water from a warm ocean or powerplant outlet, for example.
Additional methods of desalination are multiple effect distillation (“MED”) or multi-stage flash (“MSF”). MED and MSF desalinate saltwater by repeatedly evaporating and condensing the saltwater over a series of multiple stages. The source of the energy for MED and MSF processes is usually low pressure steam. The primary drawback of MED and MSF processes is the large amount of thermal energy these processes consume, which is typically an order of magnitude higher than the electrical energy used by RO.
Another method of desalination is electrodialysis (“ED”). ED achieves desalination through a separation process whereby dissolved salt ions are transferred from a feed stream to a concentrate stream through ion exchange membranes under the influence of an externally applied electric potential. This ion transport is typically conducted using an ED stack, which is constructed using an alternating arrangement of ion exchange membranes, with feed and concentrate streams flowing between the membranes. One problem with ED is that it consumes more energy than RO for desalination of seawater, and that the source of such energy is entirely in the form of an externally applied electric potential. In addition to problems associated with energy consumption, electrical hardware in the form of a direct current power source or rectifiers to generate direct current from an alternating current power source is required. A second problem with ED is that often, as a result of the magnitude of the externally applied electric potential, voltage gradients cause salt ions to migrate not only through the ion exchange membranes as intended but also through the manifolding used in the ED stack. This results in circulating ionic current losses and reduces the efficiency of ED.
Consequently, there is a need for a method and apparatus for desalinating saltwater that improves on the prior art.